Canadian Rock, at 4 Billion Years, Is Called Oldest

By PHILIP J. HILTS, Special to The New York Times

Published: October 5, 1989

WASHINGTON, Oct. 4—
Researchers have unearthed the oldest rocks ever discovered, chunks of granite in remote northern Canada that are nearly four billion years old, the National Science Foundation announced today.

The researchers, led by Dr. Samuel Bowring of Washington University in St. Louis, pushed back the date of the oldest known rocks more than 100 million years, into a time in the history of the planet about which almost nothing is known with certainty.

''Until now, there just has been no record of the earth between the time it was formed 4.5 billion years ago, and 3.8 billion years ago,'' Dr. Bowring said. He said this find pushes back the date of the earliest rocks to 3.96 billion years ago, and preliminary evidence suggests that nearby rocks in the area may be as old as 4.1 billion years. When Crust Was Formed

The finding helps pinpoint the time when the earth's crust formed, sketching in the details between the time the earth was entirely hot, liquid rock and the formation of a crust on its surface.

Dr. Bowring and his group, working with researchers from the Geological Survey of Canada, had been sampling and mapping the geology of the rolling, treeless Arctic tundra about 250 miles north of Yellowknife, in the Northwest Territories, when they gathered samples in 1984.

Two rocks, one about five pounds and the other about 50 pounds, sat unanalyzed for years in Dr. Bowring's laboratory until this spring. Then, after preliminary analysis suggested great age, 53 small crystals from the two rocks were shipped to the Australian National University for dating by a special technique that can determine how much uranium has decayed to lead in tiny samples of rock.

Very few samples of old earth's crust have been found despite extensive searches, and so the importance of these rocks and other samples still to come from the area has grown. Formed Out of Gases

The earth was formed about 4.5 billion years ago, congealing from a spinning, gaseous mass along with the other inner planets in the solar system. The earth was at first entirely molten rock, but gradually the heavier elements, like iron and nickel, settled into a dense inner core.

Around the core was the mantle, made of lighter materials called basalt. Today's report offers information about when the mantle began to develop a crust. Scientists liken the process to the formation of a crust on top of boiling pea soup.

The least dense material, like granite, surfaced and formed clumps that were like the island chains of today's earth. As they moved about on the surface of the boiling molten rock, more and more of these surface clumps collided, eventually building up into the thicker continental plates, Dr. Bowring said.

The earth's crust is now thickest under the continental plates and thinnest under the ocean, where new crust is still welling up at oceanic ridges from the underlying hot mantle. The question that may eventually be resolved by samples like those announced today is whether a crust formed very slowly over the first two billion years of the planet's history and little has remained from the early times, or whether crust was formed early, but later destroyed by widewpread meteoric bombardment of the earth.

''We didn't go out expecting to find rocks this old; in fact I had predicted that the samples we got would be young,'' said Dr. Bowring. ''There is nothing about these rocks that looks any different or that says old rocks. But now the potential for new discoveries in this area is large.'' He said he had recently hauled back more than 4,000 pounds of rocks from the Arctic wilderness for analysis.